This invention relates generally to devices and methods for making temperature measurements, and more specifically to devices and methods that make such measurements by optical techniques that utilize temperature-sensitive phosphors.
There are many methods currently used for temperature measurement. The most common industrial techniques utilize thermocouples, thermistors or resistance thermometers by means of which electrical signals are generated and then converted into temperature readings or employed for control functions.
On occasion, however, it is useful, and sometimes essential, to obtain temperature data by non-electrical techniques. This may occur: (1) where temperatures over large areas are to be measured and measurement by a dense distribution of thermocouples thus becomes impractical; (2) where the attachment of thermocouples and leads would alter the temperatures to be measured; (3) in environments where, because of high electric or magnetic fields, metallic wires are undesirable; (4) where electrical isolation and/or insensitivity to electrical noise generation is desired; (5) where, because of motion or remoteness of the part to be sensed, permanent lead wires are impractical; or (6) where, because of corrosive chemical environments, wires and thermocouple junctions would be adversely affected, with resultant changes in electrical characteristics. In these situations, optical techniques frequently become preferable.
The most direct optical technique for temperature measurement is infrared radiometry. However, where line of sight measurement is not possible, without infrared transmission media, the infrared techniques suffer a disadvantage. In such an instance there are relatively few materials sufficiently transparent to long-wave infrared radiation to provide an infrared conducting path from the area where temperature is to be sensed to the infrared detector. Furthermore, infrared techniques are not absolute in that the emissivity of the emitting material has to be known accurately if the infrared radiometric measurements are to be converted into true temperature readings.
Optical pyrometers can also be used, but only for very hot sources which emit visible radiation. Optical pyrometers also suffer from the same problems as infrared radiometers when it comes to absolute measurements.
For large area measurements, thermographic phosphors or liquid crystals are sometimes employed in the form of films, paint or coatings applied to the surface to be measured. Known typical thermographic phosphors exhibit a broad fluorescence under ultraviolet excitation, this fluorescence being strongly temperature-dependent with regard to emission intensity. The fluorescent intensity of this emission "quenches" sharply as the temperature rises over a fairly narrow temperature range. It is difficult to calibrate a thermographic phosphor absolutely because changes in excitation, such as might be caused by source instability, can be misinterpreted as a temperature variation. Liquid crystals change their reflected colors with temperature over a similarly narrow range. Both materials suffer from the fact that, to achieve high sensitivity, the range over which the materials will operate as temperature sensors is of necessity fairly restricted compared with the materials of this invention. Most liquid crystal materials are also relatively unstable and may change their chemical and physical properties over a period of time. While this is not always a problem, it can be in selected applications.
Therefore, it is a primary object of the present invention to provide techniques for remote temperature measurement using optical rather than electrical techniques that permit elimination of metallic wires, junctions and connectors, that circumvent electrical noise sources and that provide for measurement over extended areas as well as point measurements.
It is another object of the present invention to provide an internally calibrated phosphor temperature measuring system whereby changes in total fluorescent intensity with time as might be caused by a variation in excitation, changes in optical transmission with time or changes in sensitivity of a receiving detector with time are not interpreted as temperature changes.
It is yet another object of the present invention to provide a means of measuring temperatures of objects or environments without the necessity of direct physical contact with electrical wires, such as situations where the point to be measured is submerged in a corrosive gas or liquid, must be isolated electrically or thermally, is in a vacuum, or is located on a moving part to which permanent leads cannot be conveniently connected.
Finally, it is an object of the present invention to provide a means of making absolute, internally calibrated temperature measurements over wider temperature ranges than would be possible with conventional thermographic phosphors or liquid crystals.